Field Studies on the Control of a Benzimidazole Resistant Isolate of Teladorsagia (Ostertagia) Circumcincta

Barrett, Martin (1997) Field Studies on the Control of a Benzimidazole Resistant Isolate of Teladorsagia (Ostertagia) Circumcincta. PhD thesis, University of Glasgow.

Abstract

A three year field study was conducted at Moredun Research Institute's Firth Mains farm on five 0.9 hectare paddocks carrying Nematodirus battus, Trichostrongylus vitrinus and an isolate of Teladorsagia (Ostertagia) spp which was known, since 1983, to be resistant to drugs within the benzimidazole (Bz) class of anthelmintics. Ewes were randomly assigned to each paddock every year and their twin lambs treated at the manufacturer's recommended dose (MRD) for their allocated anthelmintic. A non-suppressive control regime was adopted consisting of an initial anthelmintic treatment in mid to late May to control Nematodirus and when necessary a summer drench to control other gastrointestinal nematodes. There were very few differences between the dams of the treated and control lambs with no evidence of reduced body weight, condition score or fleece weight over the three year study. There were no clinical signs of Teladorsagiosis in any of the groups of lambs throughout the study and plasma pepsinogen concentrations remained relatively low each season. There was little evidence of marked differences in performance in the fenbendazole (Fbz) treated lambs despite the presence of a Bz resistant isolate of T. circumcincta on the paddocks at Firth Mains. The maximum difference in weight gain between the Fbz and best performing group at the end of the 1993, 1994 and 1995 seasons were 3.1, 3.6 and 2.6 kgs respectively. The immunogenicity and pathogenicity of the Firth Mains Bz resistant isolate was no different to that of a Bz-susceptible or a multiple resistant isolate of T. circumcincta in a subsequent investigation. Limitations in faecal egg count reduction tests (FECRTs) and controlled efficacy tests (CETs) were apparent when dealing with naturally infected animals with differences in the acquisition and expression of immunity. Results of the FECRTs and CETs conclusively demonstrated the presence of Bz resistant T. circumcincta on the paddocks at Firth Mains and suggested that resistance to levamisole (Lev) may have developed since the start of the study. A Fbz/Lev combination remained effective with worm reductions >95 % at the end of each season. Ivermectin (Ivm) treatments were fully effective with worm reductions >99.8 % each year. Whether the expression of drug resistance that results from continued use of Bz drugs had increased over the study was very difficult to ascertain since the apparent extent of resistance, measured in terms of efficacy, was influenced by seasonally variable factors such as climate, nutrition, pasture contamination and timing of treatment. Increases in both the bioavailability and treatment efficacy of Fbz were seen as a result of feed withdrawal, divided dosing and the co-administration of piperonyl butoxide. If suitable penning is available, withholding feed offers a simple approach to maximizing the bioavailability and efficacy of Bz drugs against resistant parasites. A naturally infected lamb from each of the treatment paddocks at Firth Mains was housed towards the end of the study in 1995 and its faeces collected to provide infective larvae for artificial infection of parasite naive animals. Both the Fbz/Lev combination and the Ivm treatments were highly effective against abomasal and small intestinal species with faecal egg and worm reductions of over 99 %. The calculated efficacy for Fbz treatment using the FECRT was 77.8% whereas arithmetic and geometric mean T. circumcincta reductions were 69.9 and 70.7 % respectively. The calculated efficacy for Lev using the FECRT was 89.2% with arithmetic and geometric mean T. vitrinus reductions of 86.4 and 98.9 % respectively, the arithmetic data clearly suggesting Lev resistance in the T.vitrinus population on the paddock were Lev was employed throughout the study. A study into the use of an arbitrarily primed polymerase chain reaction (AP-PCR) in the detection of drug resistant nematode parasites sucessfully amplified DNA from individual larvae. It was unlikely that this approach could be used routinely to differentiate between susceptible and resistant parasites since there was variation in the banding patterns of single larvae from within a population. Its application as an epidemiological tool in the assessment of anthelmintic resistance however merits further study. The results of these studies are promising as regards the use of anthelmintics against which resistance has already been selected in the management of parasitic gastroenteritis, at least when it involves less pathogenic species with a low biotic potential such as Teladorsagia.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: Frank Jackson
• Keywords: Veterinary science, Parasitology
• Date of Award: 1997
• Depositing User: Enlighten Team
• Unique ID: glathesis:1997-75264
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Nov 2019 21:24
• Last Modified: 19 Nov 2019 21:24
• URI: https://theses.gla.ac.uk/id/eprint/75264